Before remodeling the interior of a house or a factory building, it is often necessary to dismantle existing ceilings and/or walls by means of a demolition cutter.
Such a demolition cutter is disclosed in JP patent 2556664. The demolition cutter disclosed in this patent includes a bracket detachably mounted to the free end of a construction machine and having a pair of side plates. An arm pin extends between the side plates of the bracket. A pair of cutter arms are pivotally mounted on the arm pin. A cylinder is connected to each of the cutter arms to pivot it about the arm pin. The cutter arms have mating surfaces formed with cutting edges. By actuating the cylinders in such directions that the cutter arms close until their cutting edges cross each other like scissors, an object sandwiched between the cutting edges is cut.
FIG. 10A is a development of the cutter disclosed in the above publication, showing the relationship between the cutter arms and their cylinders. In FIG. 10A, two arm pins 41 are shown. But this is merely because this figure is a development. The actual cutter has only one such pin 41. In FIG. 10A, as well as in FIG. 10B, the cutter arms are designated by numerals 42a and 42b, the cylinders by 43a and 43b, the mating surfaces by 44, and the cutting edges by 45.
In this conventional demolition cutter, the axes of the cylinders are present in the plane X1 including the mating surfaces 44 of the cutter arms 42a, 42b. Thus, the arm-closing forces generated by the cylinders 43a and 43b act on the respective cutter arms 42a and 42b along the plane X1. Thus, when the cylinders are actuated to close the cutter arms 42a and 42b with an object W to be cut sandwiched between the cutter arms, the cutter arms tend to pivot outwardly as shown in FIG. 10B. When the cutter arms are repeatedly pivoted outwardly, it becomes increasingly difficult to smoothly cut the object W.
The arm pin 41 is loosely inserted in pin holes 46 formed in the cutter arms 42a and 42b. That is, gaps are present between the pin 41 and the pin holes 46. Thus, when the cylinders are actuated to close the cutter arms with the object W clamped between the cutting edges 45 in an inclined state, the tips of the cutter arms 42a and 42b are pushed outwardly, so that the cutter arms 42a and 42b pivot outwardly within the range permitted by the gaps between the pin 41 and the pin holes 46 as shown in FIG. 10B. Since the arm-closing forces generated by the cylinders act on the arms along the plane X1, they cannot prevent the cutter arms from pivoting. Since the tips of the cutter arms are pushed outwardly, gaps are present between the mating surfaces 44 and thus between the cutting edges 45 when cutting the object W. This makes it difficult to cut the object W cleanly.
When the cutter arms 42a and 42b pivot outwardly about the pin 41, as shown in FIG. 11, the cutter arms are pressed hard against each other at their ends, shown at P1. The cutter arms 42a and 42b are also pressed hard against the side plates 47 of the bracket at points P2. Thus, the frictional resistance between the cutter arms and between the cutter arms and the side plates of the bracket hampers smooth pivoting motion of the cutter arms 42a and 42b in the closing directions. Also, the force from the cylinders acts on the arm pin 41 at its longitudinal center and may deflect and in the worst case break the pin.
Chips as well as a soft or thick object W to be cut tend to get stuck between the mating surfaces 44, thus hampering smooth cutting, or causing outward deformation of the tips of the arms 42a and 42b. 
When the cylinders are extended, their thrust force acts along the mating surfaces 44 of the cutter arms 42a and 42b, so that it is necessary to form the cutting edges by cutting out the mating surfaces 44. This results in a reduction in thickness of the cutter arms in their areas where the cutting edges are formed. Thus, in order to reinforce these areas, reinforcements or some other measures are necessary, which of course pushes up the cost of the entire cutter.
In order to avoid these shortcomings, in one conventional demolition cutter, pads or guide rails are provided on the outer sides of the cutter arms to prevent the arms from pivoting outwardly. But such extra members add to the number of parts and thus the cost of the cutter.
In conventional cutters, each cylinder has its body connected to the movable bracket and the piston rod to one of the cutter arms. Thus, the piston rods protrude from the movable bracket. Thus, the piston rods tend to be damaged by hitting against obstacles.
An object of the invention is to provide a demolition cutter in which the cylinders are arranged such that pushing force from the cylinders can minimize inclination of the cutter arms and outward deformation of the tips of the cutter arms when the cylinders are extended to close the cutter arms, thereby eliminating any gap between the mating surfaces of the cutter arms so that an object to be cut can be cut smoothly.
A second object of the invention is to provide a compact demolition cutter which prevents damage to the piston rods of the cylinders for opening and closing the cutter arms, and which ensures smooth demolition work.